HP Ink Jet Printer Cartridge Anatomy

Photos taken January 2002. Text written December 2002.

After I got a HP Deskjet 842C ink jet printer, and found out about the
hidden high cost of ink supplies
for it, I was naturally tempted to try to reverse-engineer the cartridges,
to find out whether the electronic ID which distinguishes the otherwise
identical expensive ripoff #15 cartridge and regular #45 cartridge (black ink)
could somehow be bypassed without disabling ink jets.

What I learned is that these cartridges are pretty fascinating technology.
I had already taken one apart and studied it visually, taking
pictures in the process, when I found US Patent
6,332,677 (the link links
directly to the USPTO, at least until they change their URL scheme). This patent
is as thorough a writeup of how these cartridges work as one could ask for.
It has 26 pages of drawings and makes good reading, despite the legalese.
The diagrams used here are taken from it. I assume this is OK, patent documents
being public material.

The following pictures show the disassembly of a #45 black cartridge.
Despite being empty, as far as the printer was concerned, it still had lots
of ink in it. I dealt with this by performing all disassembly under running
tap water in the laundry tub. This way my hands did not get stained.

After removing the metal side covers, this is what you see:

The ink compartment's side walls are a tough plastic foil, and get sucked
inward as the ink is used up. There is never any air in the ink compartment.
As you can see, the green ink level indicator is just two overlapping pieces
of cardboard, one with a window in it, the other with a green bar, which get
pulled apart as the foil moves inward.

As soon as the ink compartment is punctured, the foil sides pop apart
as air is sucked in. The reason is this spring arrangement inside:

Compressing this between your fingers, you feel a surprising amount of
force. Why is it necessary? You would think with so much force pushing
the sides of the ink pouch apart, the ink would get sucked in, not spray
out. As it happens, ink jet printheads need a certain amount of vacuum
to work properly, otherwise they dribble. The absence of this vacuum
causes improperly refilled ink cartridges to make such a mess.

The vacuum is not very strong, when you consider the area over which the
spring force is applied. In fact, if you work it out, it is only
a little more than what's needed to hold up a column of ink the height
of the cartridge when it is standing upright in the printer. Because
the ink pouch is compressed as it empties, the height of this column
never changes, and so the pressure at the print head stays the
same over the life of the cartridge.

Before reaching the print head, the ink has to pass through one of
two very fine metal screens:

I am guessing these exist so that minor clumps in the ink
can't clog up the extremely tiny ink nozzles.

Glued to the outside of the cartridge body is this flexible circuit:

It interconnects the pads that mate with the printer's contact pins
(left end) and the actual ink jets (right end). This circuit completely
covers the bottom end of the cartridge, and thus has functions not just
electrical but hydraulic as well - it forms part of the ink chamber,
and it contains the 300 tiny holes through which the ink is squirted out.

The actual works are contained on this beautiful piece of silicon:

The short sides are where it connects to the flex circuit (you can
see the holes torn into the flex when it was pulled off the chip).
The long sides contain the ink jets.

Like the flex circuit, this chip does not just function electrically but
hydraulically as well. Here is a crude microphotograph - taken with
my point-and-shoot digital camera and a cheap plastic-lens microscope - of a small part
of one of the long edges:

You can see that little "fiords" have been etched into the
chip, through which the ink can flow in from the side (remember, the top
is bonded to the flex circuit). Each ink jet chamber has a thin metal
layer plated to the bottom of it, forming a resistor. When this resistor
is suddenly heated with a large current, a layer of ink directly above
it vaporizes, sending a shock wave upward. This shock wave causes
a droplet of ink to be expelled through the hole in the flex circuit.
The following two diagrams show this better:

This may seem simple, but enormous precision is required. For example,
alternating rows of the 600dpi resolution are formed by ink jets on
opposite sides of the chip. If the chip or the flex circuit was tilted
by the tiniest fraction of a degree, the rows would not be evenly spaced.
The holes themselves must be very accurately dimensioned and shaped to
yield uniform sized droplets.

Overall, this is an impressive system. The entire printing mechanism
consists of just a silicon chip and an interconnect flex, and yet it
is able to lay down a 1/2 inch swath of printing at 600dpi in a single
pass. The electronics are liquid-cooled - by the ink, in which the
silicon chip is completely immersed.

The 300 ink jets are electrically connected as a 22x14 matrix (not all 308 positions
are used). Up to 14 jets at a time are fired, in 22 phases. To get
everything to line up vertically, given the staggered timing, the
nozzles themselves are staggered physically. HP calls the 22 bank
selects "address selects" and the other dimension "primitive selects".
Each ink jet's circuitry consists of the driving transistor and the
heater resistor, as shown:

The 14 primitive selects, 14 primitive grounds, and 22 address selects
form the bulk of the electrical connections to the cartridge.

When the cartridge is manufactured, after it has been filled with ink,
the fill hole is plugged with a steel ball:

It is impossible to extract this ball without damaging the fill hole,
so when cartridges are refilled, another plug is required. As you know
now, this plug needs to be perfectly airtight, and the ink reservoir
must have no air bubbles, for the ink pressure to be correct. I can
imagine a refill procedure that would work: Drill a hole, fill the cartridge
completely with ink (with a syringe), seal the hole up perfectly (with
glue and a screw perhaps) and then suck a sufficient quantity of ink out
of the cartridge to start compressing the spring inside. You would only
need to set the cartridge on a paper towel to do this.

Oh, and how does the printer tell a #15 cartridge from a #45? I couldn't
find any documentation on that (big surprise). But US Patent 6,325,483 describes an improvement to a cartridge ID system
that is likely the one employed. Basically, whenever one of the 22 banks
of ink jets is selected, one of 22 ID bits is also selected, and returned
through a single "sense" pin to the printer. The bad news is,
without modifying the chip, or modifying the printer (for example, by wiring
in a fake ID circuit) you cannot spoof it. As it happens, by masking
certain address lines, you can modify the ID enough to make a HP 842 ripoff
printer accept the #45 cartridge. But then you lose 14 of 300 ink jets.

A Colour Cartridge

The colour cartridges work pretty much the same as the black cartridges,
except of course the print head is more complex - three independent ink
supplies, and the chip has three slots, with ink jets on both sides of
each, for a total of six banks of jets.

The ink pressure regulation is simpler: Instead of the spring-loaded
pouch, the ink chambers simply contain a block of foam. Due to capillary
action, the pressure of ink at the bottom is always the same regardless
how much ink is still in the foam. If we pop the sides off a colour
cartridge, we can see the outer blocks of foam:

To get to the middle one, we need to remove the top center piece of
plastic:

These cartridges are much simpler to refill than the black ink ones. One
must merely remove the rubber fill plugs at the top, re-soak the foam with
a syringe until it is saturated, and put the plugs back in. The cartridge
will even work with the plugs left out.